With the arrival and expansion of the Information Highway, telephone networks have been slowly converted from an all-analog environment to a virtually all-digital network. Within these networks, the trunks and switches have been virtually 100 percent converted. However, the local loops leading to the customer remain largely analog, specifically the analog loop connecting the Central Office (CO) of the Public Switched Telephone Network (PSTN) to the subscribers' CPE.
Within the traditional telephone networks, a copper loop (or two-conductor cable), known as the subscriber loop, connecting the CO and the CPE is used to provide the POTS, whose signals are Voice Frequency (VF) signals in the frequency range of 0-4 kHz. The subscriber loop is capable of carrying signals up to several MHz, depending on its length and type. The two conductors of the subscriber loop are referred to as the TIP and RING, providing Plain Old Telephone Service (POTS) at both the subscriber premise and the CO.
Most CPEs draw DC current from the subscriber loop that connects them to the CO. For satisfactory operation, a typical CPE requires a current in the range of 18 mA to 50 mA. This DC loop current is provided by the 52 V CO power supply, whose terminals are typically coupled to the subscriber loop via a feed arrangement including two feed resistors. The DC resistance measured between the TIP and RING of the subscriber loop (including the CPE), also referred to as external resistance, is typically in the range of 100 to 1900 ohms and depends upon the length of the subscriber loop. To provide sufficient loop current for the operation of the CPE with the longest subscriber loop, the value of the feed resistors is typically limited to 200 ohm each. In this case, the loop current is calculated to be Iloop=52 V/(400+1900) ohm=22.6 mA, which is above the minimum current required for proper operation of the CPE. In the case of a short subscriber loop, one having a resistance of 100 ohm, the loop current is Iloop=52 V/(400+100) ohm=104 mA, which exceeds the desired range of loop current for proper CPE operation. In addition, this high value of DC loop current results in an excessive amount of power dissipation and consumption by the subscriber loop feed arrangement, specifically 4.32 Watts and 5.40 Watts, respectively.
A technique that has been employed with success to limit subscriber loop current to a threshold value, and consequently limit power dissipation and consumption, is described in U.S. Pat. No. 5,333,196, which issued Jul. 26, 1994 to Jakab and was assigned to Northern Telecom Limited. The contents of this document are incorporated herein by reference. The feed arrangement limits the DC loop current on short loops to a lower value, for example 30 mA, which in turn limits the power dissipation.
While the arrangement taught by Jakab does successfully limit the loop current and reduce the level of power dissipation and consumption, there is still room for improvement, in particular in instances when more than one CPE is connected to the subscriber loop. For example, when a first CPE is connected to the subscriber loop and then an additional CPE is switched on the subscriber loop, the current demands change significantly since the external resistance of the subscriber loop abruptly changes.
The background information provided above clearly indicates that there exists a need in the industry to provide a feed arrangement for a subscriber loop with an advanced current regulation capability capable of supporting multiple CPEs that can become active at any given time.